1. Field of the Invention
The present invention is generally directed to sensors and particularly to sensors used in stress wave analysis.
2. Description of Related Art
Stress wave analysis is an ultrasonic instrumentation technique which is used for measurement of friction and shock in mechanical devices. Stress waves are in the form of high frequency structure borne sounds caused by friction between moving parts. The analysis of the stress waves involves the detection and amplification of the high frequency sounds. In addition to the high frequency sounds, other noises and vibration signals are also present, which are not directly related to the stress waves. However, these other signals can interfere with proper analysis of any stress waves emitted by a mechanical device and should be eliminated.
Past attempts at stress wave analysis have incorporated specially selected piezoelectric accelerometers as stress wave sensors. However, these transducers are not specifically designed to detect stress waves, and suffer important shortcomings relative to Analog Signal Conditioning (xe2x80x9cASCxe2x80x9d) and Digital Signal Processing (xe2x80x9cDSPxe2x80x9d) elements of stress wave analysis instrumentation, such as those shown in FIG. 1.
An accelerometer, when used as a stress wave sensor, is often selected to have maximum repeatability of its primary resonant frequency between 30 Khz and 40 Khz, and its sensitivity at the primary resonant frequency. When secondary resonances are also present in the sensor""s frequency response, they are often very difficult, if not impossible, to eliminate or control, with the same precision as the primary resonance as shown in FIG. 2. Efforts to adjust or control these secondary resonances may also cause unintended and undesirable changes in the sensitivity of the primary resonance.
Accordingly, what is needed in the art is a sensor having characteristics that receives stress wave signals while discarding background noises and vibrations. It is therefore to the effective resolution of the shortcomings of the prior art that the present invention is directed.
The present invention relates to a sensor having characteristics designed specifically for detecting stress waves for use in a stress wave analysis system. In order to eliminate vibration, audible noise and acoustic emission sources that are not directly related to friction and mechanical impact events in operating machinery, it is preferred to detect stress waves in a narrow frequency range, such as, but not limited to, 35 Khz to 40 KHz. At this frequency range, stress waves from friction and impact sources typically propagate through machine structures at detectable amplitudes. Thus, in order to maximize the signal to noise ratio of stress waves, relative to background noise and vibration, the sensor of the present invention is designed and calibrated with a frequency response and damping characteristics that are specifically tailored for stress wave analysis.
The sensor of the present invention preferably satisfies the following three criteria:(a) has a resonant gain of approximately 30 db, at its primary resonant frequency, to assure adequate selective amplification of stress waves;(b) provide a total energy content of the Resonant Energy Integral within a specified tolerance band (i.e. +/xe2x88x9210% of a standard value) and which can be measurable using standard test equipment and fixtures to produce calibration data that is traceable to recognized standards; and(c) have its resonant output decay to half amplitude in five cycles or less, and be down to no more than twenty (20%) percent of the initial response in the number of cycles that correspond to the corner frequency of the band pass filter.
Thus, a sensor is provided for detecting stress waves for use in a stress wave analysis system. The stress waves are preferably detected in a narrow frequency range of 35-40 KHz. At this range, stress waves from friction and impact sources typically propagate through machine structures at detectable amplitudes. In order to maximize the signal to noise ratio of stress waves, relative to background noise and vibration, the sensor of the present invention is designed and calibrated with a frequency response and damping features that are specifically tailored for stress wave analysis. The sensor preferably satisfies the following three criteria: (a) has a resonant gain of approximately 30 db, at its primary resonant frequency, to assure adequate selective amplification of stress waves; (b) provide a total energy content of the Resonant Energy Integral within a specified tolerance band and which can be measurable using standard test equipment and fixtures to produce calibration data that is traceable to recognized standards; and (c) have its resonant peak amplitude output decay to half amplitude by five cycles, and be down to no more than twenty percent of the initial response in the number of cycles that occur during the time period that corresponds to the corner frequency of a low pass filter.
Accordingly, it is an object of the present invention to provide a sensor having a frequency response and damping characteristics specifically designed for stress wave analysis.
In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.